Fiber optic sensors have become increasingly popular in multi-channel or long distance sensing systems because of their relative immunity from electromagnetic interference, intrinsic safety, and high reliability. A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in an optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the optical fiber. Changes in the temperature or applied strain will alter either the Bragg period or the refractive index of the Bragg grating; this in turn causes the wavelength at which the fiber Bragg gratings reflect to change.
FBGs (fiber Bragg gratings) have intrinsic advantages, such as accurate measurement of wavelength shifts. A large number of fiber Bragg grating sensors can be grouped into a 3D-matrix multi-channel sensing system. In such a sensing system, the number of involved sensors can vary depending on the measurement range and the number of channels of the optical switches used.
Traditional techniques to interrogate this sophisticated sensing system include wavelength-, time-, and frequency-division multiplexing (WDM, TDM, FDM), or a combination thereof. By using multiple wavelengths of optical signals carried over a single fiber, each carrying a separate channel corresponding to an optical sensor, WDM technique enables significant increase in the processing ability and decrease in the interrogation cost. TDM technique allows the interrogation of multiple optical sensors using a modulated pulse, and by tuning the group delay, the interrogation ability is enhanced. Similar to the TDM technique, the FDM technique employs frequency modulation and demodulation to enhance the interrogation ability.
Though these are well-established interrogation techniques, their limitations are also well recognized. The WDM technique requires that sensors spectra must not overlap; TDM and FDM techniques both involve a high-speed pulse modulator and frequency modulator, as well as a complicated high-bandwidth measurement system.